1. Field of the Invention
This invention relates to a catalyst temperature control system for internal combustion engines, and more particularly to a catalyst temperature control system for use in an internal combustion engine provided with a catalytic converter (catalyst) arranged in the exhaust system of the engine for purifying noxious components in the exhaust gases, and an exhaust gas recirculation system for returning part of the exhaust gases from the exhaust system to the intake system.
2. Prior Art
Generally, in order to improve the exhaust gas purifying ability of an internal combustion engine, an exhaust gas purifying device comprising a catalyst is provided in the engine for reducing the amounts of noxious components in exhaust gases emitted from the engine. As such a catalyst, a three-way catalyst is generally employed, which purifies three noxious components, i.e. CO, HC, and NOx in the exhaust gases at the same time. To obtain the best conversion efficiency (exhaust emission purifying efficiency) of the three-way catalyst, the air-fuel ratio of a mixture supplied to the engine is feedback-controlled in response to an output from an exhaust gas ingredient concentration sensor arranged in the exhaust system, so as to make the air-fuel ratio equal to a stoichiometric air-fuel ratio.
However, although the above method of feedback-controlling the air-fuel ratio has the advantage that the three-way catalyst can have a good exhaust emission purifying efficiency without requiring supply of secondary air into the exhaust system, it has the disadvantage that when the engine is operating in a high speed partial load region, the bed temperature of the catalyst can rise to an abnormally high level, resulting in deterioration of the catalyst or burning of same.
To overcome this disadvantage, a control system has been proposed e.g. by Japanese Provisional Patent PubIication Kokai) No. 52-153030, which enriches the air-fuel ratio of a mixture supplied to an internal combustion engine to a richer value than a stoichiometric air-fuel ratio so as to lower the catalyst temperature.
According to the proposed control system, by thus enriching the air-fuel ratio to a richer value than the stoiohiometric air-fuel ratio, the three-way catalyst is placed in a reducing atmosphere due to an increased amount of unburnt ingredients in the exhaust gases, whereby oxidizing reaction of the unburnt ingredients is restrained and at the same time reducing reaction (endothermic reaction) of NOx is promoted, to reduce the catalyst temperature.
However according to tests conducted by the present inventors, it has been turned out that if the catalyst is placed in a fairly high temperature state, though not so abnormally high as 800.degree. C., for a long time, it can deteriorate so that the purification rates of CO. HC, and NOx become degraded.
FIG. 1 shows deterioration characteristics of a three-way catalyst in terms of purification rates (%), obtained after endurance tests conducted at different temperatures. In the endurance tests, the vehicle in which the engine is installed was made to run over 80,000 km with the three-way catalyst held at predetermined different temperatures. In the FIGURE, the abscissa indicates the temperature at which the catalyst bed was held during the tests (.degree. C.) and the ordinate the purification rates (%) of CO, HC, and NOx. The symbol ".largecircle." represents a measuring point. The solid lines represent purification rates obtained when the air-fuel ratio is controlled to a stoichiometric air-fuel ratio (=14.7), and the chain lines purification rates obtained when the air-fuel ratio is controlled to a value (=14.3) richer than the stoichiometric air-fuel ratio.
As is clear from the FIGURE, certainly the purification rate of each of CO, HC, and NOx can be improved to some degree by controlling the air-fuel ratio to a richer value than a stoichiometric ratio. However, if the catalyst is continuously placed under a high temperature state where the catalyst bed temperature is 700.degree. C. or higher, for a long time, the purification rates of CO and HC become lower as the temperature of the catalyst bed rises. That is, even if the catalyst bed temperature is 700.degree. C.-800.degree. C., the catalyst becomes deteriorated and hence its purification rates become lower after it has been long placed under such a high temperature state.
Therefore, the above proposed control system has the disadvantage that even if the air-fuel ratio is controlled to a richer value than a stoichiometric air-fuel ratio, the catalyst becomes deteriorated after being continuously placed under a normal engine operating condition in which the catalyst bed temperature is in a range from 700.degree. C. to 800.degree. C., for a long time, resulting in lower purification rates and hence degraded purifying ability.
Further, the proposed control system has another disadvantage that if the air-fuel ratio is controlled to a richer value than the stoichiometric air-fuel ratio, the CO component is emitted in an increased amount from the engine.